A quite limited portion of a gaseous mixture aspirated into the cylinders of an internal combustion engine flows downwardly from a space between a piston and a cylinder block and accumulates in the crank case. This outflowing gaseous mixture is termed blow-by gas.
In order to combust the blow-by gas in a multi-cylinder engine, for example, the oil pan and the intake manifold of the engine are connected by a blow-by gas recirculation passage whereby blow-by gas is mixed into the gaseous mixture supplied to the cylinders using the negative intake pressure of the intake manifold. During this process, unequal distribution of blow-by gas to the respective cylinders results in deviations in the air-fuel ratio of each cylinder and has adverse effects on combustion and exhaust gas composition.